//SamXIAO
#include <bits/stdc++.h>
using namespace std;

char s1[1000][503];
int f[1086];

void w1()
{
	int k, ans = 0, cnt=0, n, ma = -999999;
	string ss;
	cin >> k;
	cin >> ss;
	int l = ss.length();
	char chPrev = -1;
	bool isFound = false;
	for(int i = 0; i < l; i++){
		char ch = ss.c_str()[i];
		if( ch == chPrev ){
			if(++cnt == k){
				printf("%c", ch);
				isFound = true;
				break;
			}
		}
		else cnt = 1;
		chPrev = ch;
	}
	
	if(!isFound) printf("No");
}

int main()
{
	w1();
  return 0;
}
